1. Field of the Invention
The present invention relates to a printer provided with an impact type print head and a platen located opposite to the print head. To be more specific, the invention relates to the structure of a platen which receives an impact from a print head at the time of printing on a print medium.
2. Description of the Related Art
For example, an impact type printer comprises a print head which is linearly moved over a print medium, and a platen which supports the print medium. The print head has a number of wires projecting toward the print medium. When the wires hit an ink ribbon upon the print medium, characters, marks or the like are printed on the print medium. The platen extends in the moving direction of the print head, and is located opposite to the print head with respect to the print medium. The platen is hit by the wires of the print head at the time of printing on the print medium. Thus, the platen is required to have a sufficient strength to withstand the impacts from the print head.
It should be noted that a passbook which is an example of a print medium is thicker than an ordinary recording paper sheet, and it has a print region whose thickness is not uniform. The print head prints a row of characters, marks or the like each time it performs printing while being moved in its width direction. Thus, if the print medium has a print region whose thickness varies as in the passbook, the distance between the print head and the print medium varies as the print head is moved to print the print medium. As a result, the print density of a thick part of the print medium is different from that of a thin part of the print medium. Inevitably, the print quality is low.
In view of the above circumstances, a printer which handles a thick print medium such as a passbook uses a print head provided with rollers for pushing the print medium against the platen. In addition, both ends of the platen are elastically supported by respective springs. Thereby, the platen is displaced in a direction toward or away from the platen in accordance with the position of the print head. As a result, the positional relationship between the print head and the print medium is constant, thus improving the print quality.
Also, in a conventional printer, both ends of the platen are supported by respective springs as in the above printer. In this structure, the pressure acting on the center of the platen toward the print head is double that on each end of the platen. In other words, the reactive force of the platen against the wires of the print head is locally high, i.e., that of the center of the platen is great. Inevitably, the print density of the part of the print medium which corresponds to the center of the platen is high, and that of the parts of the print medium which correspond to the ends of the platen is low.
In order to solve such a problem, a conventional impact printer uses a flexible platen. This platen is formed of a material having an elastic modulus, and its height increases from either of its ends toward its center. The moment of inertia of a horizontal surface extending through the center of gravity of the platen increases from either of the ends toward the center. An impact printer having such a platen is disclosed in, e.g., U.S. Pat. No. 5,879,091.
In the impact printer disclosed in the U.S. patent, although the both ends of the platen are supported by respective springs, the reactive force of each of the ends of the platen is equivalent to that of the center of the platen. This feature thus prevents increasing of the print density of part of a print medium which corresponds to the center of the platen.
However, if a platen is easily warped such that the reactive forces of the ends of the platen are equivalent to that of the center of the platen, the reactive forces of parts of the platen which are located between the center of the platen and the ends of the platen, respectively, are increased.
Consequently, when a print head reaches each of the above parts of the platen which are located between the center of the platen and the ends of the platen, the print medium is strongly pushed against the rollers of the print head. Thus, especially if a pressure sensitive copying paper sheet is used as a print medium, the marks of the rollers are transferred onto the paper sheet. On the other hand, if the reactive force of the platen is lowered in order to avoid the above problem, the print density of the entire print region of the print medium decreases. Consequently, printing is unclear, and the print quality drops.
The object of the present invention is to provide a printer in which the reactive force of the platen is made uniform over the entire length thereof, and the print quality is improved.